Recently, as a method for increasing carrier mobilities of transistors, it is proposed to apply tensile stresses or compressive stresses to the channel regions of the transistors.
For NMOS transistors, a tensile stress film, which applies tensile stresses to the channel region of the NMOS transistors, is formed, covering the NMOS transistors.
For PMOS transistors, a compressive stress film, which applies compressive stresses to the channel regions of the PMOS transistors, is formed, covering the PMOS transistors.
As a material of the tensile stress film and the compressive stress film, silicon nitride film is used.
Related reference is as follows:    Japanese Laid-open Patent Publication No. Hei 7-106325; and    Japanese Laid-open Patent Publication No. Hei 5-190540.